The present invention relates to a process for preparing tantalum alkoxides and niobium alkoxides and to their use.
Tantalum alkoxides and niobium alkoxides can be utilized for the deposition of corresponding metal layers by means of chemical vapor deposition (CVD) and are therefore valuable starting compounds for producing extremely resistant components which are employed, for example, in the electronics industry.
The preparation of tantalum alkoxides and niobium alkoxides starts out from the corresponding metal chlorides. Thus, the reaction of niobium(V) chloride (NbCl5) with alcohol and ammonia was described as early as 1956 by D. C. Bradley, B. N. Chakravarti and W. Wardlaw (J. Chem. Soc., 1956, 2381-2384). The reaction is carried out in two steps. Firstly, the alcohol is reacted with a suspension of NbCl5 in an organic solvent, for example, benzene, resulting in a vigorous reaction. An excess of ammonia is subsequently passed into the reaction mixture. This forms ammonium chloride which is separated off by filtration and the desired niobium alkoxide which is isolated by distillation.
A two-stage process has also been described for the preparation of tantalum alkoxides (G. Dxc3xa4ndliker, xe2x80x9cxc3x9cber Alkoxyde und Phenolate von Niob und Tantalxe2x80x9d CIBA Basle, 1962), with the alcohol firstly being reacted with tantalum(V) chloride in the presence of toluene. An excess of alcohol is used and the reaction proceeds according to the following equation:
TaCl5+ROH⇄TaCl2(OR)3+3HClTaCl(OR)4+4HCl⇄Ta(OR)5+5HClxe2x80x83xe2x80x83(1)
This is an equilibrium reaction in which the equilibrium can be shifted as a function of the HCl concentration. To obtain the desired product Ta(OR)5, ammonia gas is passed into the reaction mixture in a second step. Here too, ammonium chloride formed is filtered off and the product is worked up by distillation. For example, tantalum(V) ethoxide is prepared by reaction of tantalum(V) chloride with ethanol in the presence of toluene at 50xc2x0 C., with HCl being liberated. Ammonia gas is subsequently introduced, resulting in a rise in the reaction temperature to 64xc2x0 C.
The processes described have the disadvantage that HCl gas is liberated, as a result of which an apparatus in which the reaction is carried out is subjected to severe corrosion. In addition, the reaction is carried out using large amounts of organic solvents which, in the preparation of niobium alkoxides and tantalum alkoxides on an industrial scale, have to be worked up or disposed of, which is costly. The reaction temperatures reported (50xc2x0 C.) are significantly above the flash point of ethanol (12xc2x0 C.), which is problematical from the point of view of safety.
It is therefore an object of the present invention to provide an efficient process for preparing niobium alkoxides and tantalum alkoxides which can be carried out without addition of organic solvents.
The invention relates to a process for preparing a niobium(V) alkoxide or a tantalum(V) alkoxide of the formula (I)
M(OR)5xe2x80x83xe2x80x83(I),
wherein M represents Nb or Ta and R represents C1-C5-alkyl. The process comprises dissolving NbCl5 or TaCl5 in an alcohol of the formula (II)
ROHxe2x80x83xe2x80x83(II),
wherein R represents C1-C5-alkyl, at a temperature ranging from about 0xc2x0 C. to about xe2x88x9250xc2x0 C. under an inert atmosphere, wherein the alcohol contains from about 5 to about 7 mol of ammonia per mol of NbCl5 or TaCl5, and reacting NbCl5 or TaCl5 with alcohol of the formula (II), and thereby forming the niobium(V) alkoxide or the tantalum(V) alkoxide of the formula (I).
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
The invention provides a process for preparing niobium(V) alkoxides and tantalum(V) alkoxides of the formula (I)
M(OR)5xe2x80x83xe2x80x83(I),
where
M represents Nb or Ta and
R represents C1-C5-alkyl,
by reacting NbCl5 or TaCl5 with an alcohol of the formula (II)
ROHxe2x80x83xe2x80x83(II),
where R is as defined above, in the presence of ammonia, which is characterized in that NbCl5 or TaCl5 is dissolved at a temperature of from about 0xc2x0 C. to about xe2x88x9250xc2x0 C. under an inert atmosphere in the alcohol of the formula (II) containing from about 5 to about 7 mol of ammonia per mol of NbCl5 or TaCl5 to be reacted.
The process of the invention is also suitable for preparing a mixture of niobium(V) alkoxides and tantalum(V) alkoxides of formula (I).
The process of the invention allows the preparation to be carried out without liberation of HCl gas and without use of large amounts of organic solvents. Furthermore, despite the fact that the reaction is carried out at a comparatively low temperature, it surprisingly enables the reaction time to be significantly reduced.
A particular advantage is that the process of the invention can be carried out without liberation of HCL gas. In one embodiment, the process liberates less than about 0.5 mole HCL gas per mole NbCl5 or TaCl5, preferably less than about 0.1 mole HCL gas, more preferably less than about 0.05 mole HCL gas. More preferably, the process according to the invention does not liberate any HCL gas.
Advantageously, as mentioned above, the process of the invention is carried out without use of large amounts of organic solvents. In one embodiment, less than about 0.1 mole of an organic solvent per mole of alcohol is used, preferably less than about 0.05 mole organic solvent is used, more preferably less than about 0.01 organic solvent is used. More preferably, the process according to the invention is carried out without use of any organic solvents other than the alcohol employed.
The process of the invention is particularly suitable for preparing niobium(V) alkoxides and tantalum(V) alkoxides of the formula (I) in which R preferably represents methyl, ethyl, n-propyl, i-propyl, n-butyl-, i-butyl or n-pentyl. R particularly preferably represents ethyl. M in the formula (I) preferably represents tantalum.
The preparation of the compounds niobium(V) chloride and tantalum(V) chloride used in the process of the invention is known to those skilled in the art. The preparation of NbCl5 is, for example, described in J. Chem. Soc., 1956, p. 2383.
The reaction is preferably carried out at a temperature ranging from about xe2x88x9210xc2x0 C. to about 45xc2x0 C., particularly preferably at a temperature ranging from about xe2x88x9225xc2x0 C. to about 40xc2x0 C.
NbCl5 or TaCl5 is preferably reacted with an alcohol of the formula (II) containing from about 5.5 to about 6 mol of ammonia per mol of NbCl5 or TaCl5 to be reacted.
The alcohol containing the desired amount of ammonia can be prepared by passing a defined amount of dry ammonia into the appropriate amount of cooled alcohol with the aid of a glass frit.
Ammonia is preferably used in the form of ammonia gas having a purity of 99.9%. Relatively large traces of water in particular have an adverse effect on the desired reaction. It is therefore advantageous to dry the ammonia gas, for example, by passing the ammonia over solid sodium hydroxide, before it is passed into the alcohol.
It is advantageous to use from about 80 to about 90 moles of alcohol per mole of NbCl5 or TaCl5. It is possible to circulate unreacted alcohol. Preference is given to using alcohol having a water content of less than about 0.1 mol %.
Although the reaction of the invention is preferably carried out without addition of a further organic solvent, such an addition is nevertheless possible. An example of a suitable solvent is heptane.
The reaction is carried out in an inert atmosphere. For example, the reaction can be carried out in the presence of nitrogen or a noble gas.
The reaction mixture is preferably worked up by filtering off the ammonium chloride at a temperature ranging from about xe2x88x9225xc2x0 C. to about xe2x88x9235xc2x0 C., distilling the alcohol from the filtrate, filtering again at a temperature ranging from about 5xc2x0 C. to about 0xc2x0 C. and subsequently distilling the product under reduced pressure.
If tantalum(V) ethoxide is prepared by the process of the invention, it is advantageous to add small amounts, for example, about 1 per mil (0.1% by weight), of sodium methoxide after the second filtration at from about 5xc2x0 C. to about 0xc2x0 C. and to carry out a double distillation under reduced pressure.
Niobium and tantalum alkoxides prepared according to the invention can be used, for example, as starting compounds in chemical vapor deposition (CVD) processes.
Furthermore, the niobium and tantalum alkoxides prepared according to the invention can be employed in the preparation of catalysts, for the deposition of thin niobium oxide, tantalum oxide, niobium nitride or tantalum nitride films from solutions or for the deposition of homogeneous niobium or tantalum coatings by means of spin coating, dip coating or sol gel coating.
As such, in one embodiment, Applicants"" is a process comprising subjecting, to a chemical vapor deposition process, a niobium(V) alkoxide or a tantalum(V) alkoxide of the formula (I) M(OR)5 wherein the niobium(V) alkoxide or a tantalum(V) alkoxide of the formula (I) is prepared by dissolving NbCl5 or TaCl5 in an alcohol of the formula (II)
ROHxe2x80x83xe2x80x83(II),
at a temperature ranging from about 0xc2x0 C. to about xe2x88x9250xc2x0 C., under an inert atmosphere, wherein the alcohol contains from about 5 to about 7 mol of ammonia per mol of NbCl5 or TaCl5, reacting NbCl5 or TaCl5 with alcohol of the formula (II), and thereby forming the niobium(V) alkoxide or the tantalum(V) alkoxide of the formula (I).
In another embodiment, Applicants invention is a process comprising depositioning, to a substrate, a niobium(V) alkoxide or a tantalum(V) alkoxide of the formula (I) M(OR)5 (I), wherein M represents Nb or Ta and R represents C1-C5-alkyl; wherein the niobium(V) alkoxide or a tantalum(V) alkoxide of the formula (I) is prepared by dissolving NbCl5 or TaCl5 in an alcohol of the formula (II)
ROHxe2x80x83xe2x80x83(II),
wherein R represents C1-C5-alkyl, at a temperature ranging from about 0xc2x0 C. to about xe2x88x9250xc2x0 C., under an inert atmosphere, wherein the alcohol contains from about 5 to about 7 mol of ammonia per mol of NbCl5 or TaCl5, reacting NbCl5 or TaCl5 with alcohol of the formula (II), and thereby forming the niobium(V) alkoxide or the tantalum(V) alkoxide of the formula (I).
In another embodiment, Applicants"" invention is a process comprising depositioning (by spin coating, dip coating or sol gel coating) a niobium(V) alkoxide or a tantalum(V) alkoxide of the formula (I)
M(OR)5xe2x80x83xe2x80x83(I),
wherein M represents Nb or Ta andR represents C1-C5-alkyl; to a substrate and forming a film selected from the group consisting of niobium oxide films, tantalum oxide films, niobium nitride films from a solution, wherein the niobium(V) alkoxide or a tantalum(V) alkoxide of the formula (I) is prepared by dissolving NbCl5 or TaCl5 in an alcohol of the formula (II)
ROHxe2x80x83xe2x80x83(II),
wherein R represents C1-C5-alkyl, at a temperature ranging from about 0xc2x0 C. to about xe2x88x9250xc2x0 C., under an inert atmosphere, wherein the alcohol contains from about 5 to about 7 mol of ammonia per mol of NbCl5 or TaCl5, reacting NbCl5 or TaCl5 with alcohol of the formula (II), and thereby forming the niobium(V) alkoxide or the tantalum(V) alkoxide of the formula (i).